Method and apparatus for monitoring and controlling characteristics of process effluents

ABSTRACT

A method and an apparatus for monitoring and controlling a characteristic of process waters or effluents from wood pulp bleaching, pulping and paper making processes utilizing a bleaching agent such as hydrogen peroxide (H 2  O 2 ), Na 2  S 2  O 4 , ClO 2 , Cl 2  or O 3  or a pulp delignification process utilizing a delignification agent such as NaOH, Na 2  S, O 2 , Na 2  SO 3 , and enzymes including ligninase, xylanase, mannanase, laccase, and peroxidase are disclosed. This is done by obtaining at least three measurements of ultraviolet-visible light from the effluent by taking a first measurement measured at a first wavelength, a second measurement measured at a second wavelength, and a third measurement at a third wavelength, formulating two ratios from the three measurements and using the ratios for computing an empirical value of the characteristic of the effluent. Feedback control is used for adjusting feed input components in accordance with the computed empirical value of the characteristic such that a target measurement of the characteristic is obtained.

FIELD OF THE INVENTION

The present invention relates to the monitoring and controlling of aprocess using ultraviolet light absorption. In particular, the inventionrelates to the monitoring and controlling of a wood pulp bleaching,pulping or paper making process by using UV light absorption ratios ofthe process effluent.

BACKGROUND OF THE INVENTION

In many industrial chemical processes, the amount of reactants, or inputcomponents, that are used is less than or more than the amount necessaryto carry the reaction to the point of obtaining a desiredcharacteristic(s) of the product stream. If too little of the inputcomponent is used, often the desired target value of a characteristicfrom the process is not obtained. Alternatively, if an excessive amountof an input component is used, the desired characteristic may beobtained, but the excess input component is typically released as wastein the effluent of the process. In other cases, excessive amounts of aninput component may cause undesirable reactions to occur that produceunwanted characteristics. Further, the wasted input component iseconomically costly and can become an environmental pollutant if it isreleased into the environment without being removed or recycled from theeffluent.

The difficulty in controlling chemical processes, such as bleaching, inthe pulp and paper manufacturing industry can be caused by a number offactors including qualitative and quantitative variability of the pulpor wood furnish, the composition of the process chemicals, and theconsistency (% wood or pulp) of the furnish. Further, changing marketrequirements for paper products may require a paper manufacturingoperation to produce a wide variety of paper grades. New paperprocessing methods, equipment, and chemicals force the paper bleachingoperation to adapt to these technical changes while still monitoringvarious characteristics of the pulp.

It is therefore desirable to be able to precisely control the inputcomponents to obtain the desired target characteristic(s) with littlewaste. To obtain this control, a characteristic of the effluent of anindustrial process should be precisely monitored in real time in orderto provide feedback control on the amount of input components whichshould be added to the reactor to avoid under use or excessive use, andwaste, of the input component.

For example, in the pulp and paper industry, hydrogen peroxide and thehydroperoxy anion (HO₂ ⁻) are important input components for theoxidation and bleaching of wood pulps. In a typical pulp bleaching plantsituation, the control of the bleaching chemicals is based on thebrightness of the incoming pulp, the pulp flow, and the targetbrightness that is to be achieved. The factors of incoming pulpbrightness, pulp flow, and target brightness are then used to calculatethe amount of bleaching chemicals required to be added to the pulp toachieve a certain final target brightness. In another system, thebrightness of the pulp is measured after bleaching chemicals are addedand after allowing the reaction to occur for a defined reaction time.The resultant brightness value of the reaction is then measured and isused for feedback regulation of the bleaching chemicals.

Typically with these feedback systems, the amount of hydrogen peroxidethat is used exceeds or overshoots the amount necessary to reach a finaltarget characteristic, such as pulp final target brightness, yellowness,residual peroxide, brightness efficiency, yellowness efficiency, anddelignification efficiency. The resultant unwanted variation in thesepulp characteristics may cause additional processing problems in thepulp and paper processing mill. Further, in the case of peroxidebleaching, excessive use of hydrogen peroxide results in waste hydrogenperoxide in the pulp effluent, which is both costly and environmentallyharmful.

In order to solve these problems the prior art has offered varioussolutions. For example, U.S. Pat. No. 4,878,998 teaches a method forbleaching of mechanical, thermomechanical and chemi-mechanical pulpswhereby peroxide bleaching is controlled by the addition of a presetamount of bleaching chemicals at a first bleaching stage, measuring thebrightness of the pulp, feed forwardly adjusting the amount of bleachingchemicals to be added at a second bleaching stage as a function of themeasured brightness of the pulp from the first stage, and then bleachingthe pulp at the second stage.

Canadian Patent No. 2,081,907 teaches a method and apparatus fordetermining information characteristics of the concentration of each ofat least three intermixed components in kraft liquors having the stepsof identifying detectable characteristics that are detectable inrelation to the concentration of the components, developing amathematical relationship between the component and the characteristics,such as regression analysis, analysing a sample of solution with a LTVdetector, and then controlling the concentration of each of the threecomponents by using the information from the analysis of the sample.

While current brightness sensors are able to provide a measure of thepulp brightness, they are unable to measure the bleaching efficiency ofthe bleaching reaction itself. (Bleaching efficiency is the change inbrightness of the pulp divided by the residual peroxide in the pulpeffluent.) Further, measurement of yellowness efficiency (the change inpulp yellowness divided by the residual peroxide in the pulp effluent)also requires a method by which the residual peroxide in the pulpeffluent can be measured.

Unfortunately, it is well known that there is a present lack of anappropriate method or device for the monitoring and control of pulpbleaching reaction characteristics, including pulp final targetbrightness, yellowness, residual peroxide, brightness efficiency,yellowness efficiency, and delignification efficiency. Also, it is knownthat pH measurement probes and electrochemical methods of measuringhydrogen peroxide, such as the Kajaani Polarox sensor made by ValmetAutomation, can be unreliable under pH conditions which are typicallyused for pulp brightening reactions.

SUMMARY OF THE INVENTION

In one aspect of the present invention, it provides a method formonitoring a characteristic of an effluent of a process comprising thesteps of:

(a) obtaining at least two measurements of ultraviolet light absorptionof the effluent, each at a different wavelength by obtaining:

one measurement of light absorption at a first ultraviolet wavelength;and

a second measurement of light absorption at a second ultravioletwavelength; and

(b) comparing a ratio of these at least two measurements to apredetermined interrelationship of the ratio and the characteristic ofthe effluent, to determine the empirical value of the characteristic.

In another aspect of the present invention, it provides a method formonitoring and controlling a characteristic of an effluent comprisingthe steps of:

(a) obtaining at least two measurements of ultraviolet light absorptionof the effluent, each at a different wavelength by obtaining:

one measurement of light absorption at a first ultraviolet wavelength;

a second measurement of light absorption at a second ultravioletwavelength;

(b) comparing a ratio of these two measurements to a predeterminedinterrelationship of the ratio and the characteristic of the effluent,to determine the empirical value of the characteristic; and

(c) controlling the characteristic using feedback control whereby one ormore feed input components are adjusted in accordance with thedetermined empirical value of the characteristic, to obtain the desiredtarget measurement of the characteristic.

In another aspect of the present invention provides an apparatus formonitoring a characteristic of an effluent of a process comprising:

(a) a means for obtaining at least two measurements of ultraviolet lightabsorption of the effluent, each at a different wavelength by obtaining:

one measurement of light absorption at a first ultraviolet wavelength;and

a second measurement of light absorption at a second ultravioletwavelength; and

(b) a means for comparing a ratio of these at least two measurements toa predetermined interrelationship of the ratio and the characteristic ofthe effluent, to determine the empirical value of the characteristic.

In another aspect, the present inventor provides an apparatus formonitoring and controlling a characteristic of an effluent comprising:

(a) a means for obtaining at least two measurements of ultraviolet lightabsorption of the effluent, each at a different wavelength by obtaining:

one measurement of light absorption at a first ultraviolet wavelength;

a second measurement of light absorption at a second ultravioletwavelength;

(b) a means for comparing a ratio of these two measurements to apredetermined interrelationship of the ratio and the characteristic ofthe effluent, to determine the empirical value of the characteristic;and

(c) a means for controlling the characteristic using feedback controlwhereby one or more feed input components are adjusted in accordancewith the determined empirical value of the characteristic, to obtain thedesired target measurement of the characteristic.

According to an embodiment of the invention, process waters or effluentis produced from wood pulp bleaching, pulping and paper making processesutilizing a bleaching agent such as hydrogen peroxide (H₂ O₂), Na₂ S₂O₄, ClO₂, Cl₂ or O₃ or a pulp delignification process utilizing adelignification agent such as NAOH, Na₂ S, O₂, Na₂ SO₃, and variousenzymes including ligninase, xylanase, mannanase, laccase, andperoxidase.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described in more detail, withreference to the appended Figures, wherein:

FIG. 1 is a schematic diagram of a preferred embodiment of the presentinvention.

FIG. 2 is a graphical representation of the predeterminedinterrelationship of the ratio of ultraviolet light absorption at 230 nmand 300 nm (A₂₃₀ nm /A₃₀₀ nm) and the weight percentage value of H₂ O₂(as a weight percentage of dry pulp weight) of the pulp effluent.

FIG. 3 is a graphical representation of the predeterminedinterrelationship of the ratio of ultraviolet light absorption at 230 nmand 250 nm (A₂₃₀ nm /A₂₅₀ nm and the pH value of the pulp effluent fordifferent reaction times.

FIG. 4 is a graphical representation of the predeterminedinterrelationship of the ratio of ultraviolet light absorption at 280 nmand 350 nm (A₂₈₀ nm /A₃₅₀ nm) and the brightness (ISO) of the pulp afterbrightening.

FIG. 5 is a graphical representation of actual and predicted values ofthe brightness (ISO) of spruce wood pulp as a function of pH of sprucewood pulp using a multilinear regression with ratios of ultravioletlight absorption of A₂₃₀ nm /A₂₅₀ nm A₃₅₀ nm /A₂₈₀ nm and A₂₃₀ nm /A₃₀₀nm.

FIG. 6 is a graphical representation of actual and predicted values ofthe residual H₂ O₂ (peroxide) concentration in spruce wood pulp effluentas a function of pH of spruce wood pulp using a multilinear regressionwith ratios of ultraviolet light absorption of A₂₃₀ nm /A₂₅₀ nm, A₃₅₀ nm/A₂₈₀ nm and A₃₅₀ nm /A₃₀₀ nm.

FIG. 7 is a graphical representation of actual and predicted values ofthe delignification efficiency of spruce wood pulp as a function of pHof spruce wood pulp using a multilinear regression and ratios ofultraviolet light absorption of A₂₃₀ nm /A₂₅₀ nm) A₃₅₀ nm /A₂₈₀ nm andA₃₀₀ nm /A₂₈₀ nm.

DETAILED DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a pulp bleaching process utilizing apreferred embodiment of the present invention. As shown in FIG. 1, woodpulp 2 is bleached in a bleaching chamber 4, by bleaching agent hydrogenperoxide (H₂ O₂), as input component. H₂ O₂ flows from reservoir 18through control valve 16 into the bleaching chamber 4. The output of thebleaching process is a process effluent 6 which includes both thebleached pulp as well as the bleaching liquor.

A sample of the effluent 6 is diverted to an ultraviolet (UV) lightsensor 10 for the purpose of obtaining at least two measurements of UVlight absorption of the effluent 6 and then calculating the ratio of thetwo measurements. The ratio of the UV light absorbance fit measurementshave been found to correlate to various characteristics of the pulpeffluent. For example, characteristics such as pulp brightness, pH, andpulp yellowness, and residual peroxide can be determined through the useof different UV wavelengths in the ratio. It has also been found thatdifferent types of wood pulp, such as aspen or spruce, require differentUV wavelengths to measure the same characteristic of the effluent. Inparticular, it may be desirable to select alternative wavelengths whichcorrespond to the peaks and valleys of the UV absorbance spectra for aparticular characteristic of the specific effluent.

The bleaching process causes structural changes in the lignin orextractive components of the pulp including ionization of the phenolicgroups of the lignin molecule. Lignin degradation typically results inan increase in the number of phenolic groups that can be ionized bychanges in pH. As a result, the relative amount of ionized phenolicgroups usually depends on the extent of lignin degradation and pH. Ithas been found that UV light absorbance of the pulp effluent atwavelengths of 230±5 nm (A₂₃₀±5 nm), 250±5 nm (A₂₅₀±5 nm), 300±10 nm(A₃₀₀±10 nm), and 350 nm (A₃₅₀ nm) varies in relation to the degree ofionization of phenolic groups in the lignin molecule. Therefore, UVlight absorption at these wavelengths provides a means to measure acombination of the degree of lignin degradation and pH. In order tomeasure pH or lignin degradation itself, one must understand theconditions of the particular bleaching process and the pH range overwhich the variable (pH or lignin degradation) is to be measured. Thesewavelengths are most effective to measure pH at pH values which areclose to the pK_(a) values of the lignin components. For example, it ismore effective to measure pH in ranges which are close to the pK_(a) ofthe phenolic structural groups (pK_(a) =10.0), or the p-hydroxybenzoicacid groups (pK_(a) =9.0), the phenolic groups (with a carbonyl group)(pK_(a) =7.4), or a carboxylic acid group (pK_(a) =4.8). Alternatively,these wavelengths are most effective to measure lignin degradation wherethe pH is 10.5 or greater. For example, in the kraft pulping processeswhich generally occur at pH 10.5 and greater, UV measurements taken atthese wavelengths generally correlate with the extent of lignindegradation.

Further, it has been also found that UV light absorption at wavelengthsbetween 270 nm and 280 nm (A₂₇₀₋₂₈₀ nm) is invariant with the structureof the lignin molecule and provides a measure of the delignification ofthe pulp (i.e. delignification is a term referring to the removal oflignin and lignin components from the pulp.). Additionally, UV lightabsorbance at 300 nm generally varies with changes in pH.

Further, the yellowness of the pulp, caused by the existence of colouredcompounds, generally corresponds to UV light absorbance at wavelengthsof 330 nm, 350 nm, 400 nm, and 450 nm (A₃₃₀ nm, A₃₅₀ nm, A₄₀₀ nm,. andA₄₅₀ nm). Molecular structures whose presence has a high correlation toUV light absorbance at these wavelengths include quinones, ∝-carbonylphenolic structures, coniferaldehyde structures, stilbenes and ligninmetal (iron or manganese) complexes.

According to the present invention, two UV light absorbance measurementsare obtained from the pulp effluent and a ratio is calculated from thesemeasurements. In the case of pulp brightness, a ratio is calculatedusing a first measurement of UV light absorption at a wavelength ofeither 230±5 nm, 250±5 nm, 300±10 nm, and 350 nm. A second measurementis obtained at a different wavelength of 230 nm or 270 nm to 280 nm. Theratio of the first and second measurements provides an indirectmeasurement of pulp brightness by measurement of properties of the pulpeffluent: the concentration of hydrogen peroxide, the ionization of thelignin phenolic groups, and the colour of the pulp effluent relative toamount of lignin remaining in the pulp (as a result of delignification).

In the case of pulp yellowness, a ratio is calculated using a firstmeasurement of UV light absorption of the pulp effluent at a wavelengthof 330 nm, 350 nm, 400 nm, and 450 nm. A second measurement is obtainedat 270 nm to 280 nm. The ratio of these two measurements provides ameasurement of pulp yellowness by measurement of the coloured compoundsrelative to the amount of lignin remaining in the pulp (as a result ofdelignification).

Further, for measuring residual peroxide in the pulp effluent, one ofthese measurements is made at a first wavelength, such as 230 nanometers(mn), while the second measurement is made at a different wavelength,such as 280 nm or 300 nm. The ratio of these measurements provides ameasure of the amount of peroxyl anion in the pulp effluent, therebyindirectly measuring the residual peroxide in the pulp effluent.

In the case of measuring the pH of the pulp effluent, one of thesemeasurements is made at a first wavelength, 230 mn or 300 mn, while thesecond measurement is made at 250 nm or 280 nm. It is believed that asthe lignin phenolic groups are ionized, the UV absorbance for givenchromophore shifts up to longer wavelengths by approximately 20-30 nm.For example, the UV light absorption of a chromophore at 230 nm at pH 9will shift up to absorb UV light at approximately 250 nm at pH 11.Similarly, the absorbance spectra of a chromophore at 300 nm at pH 11will shift down to absorb light at 270 nm at a lower pH, as a result ofprotonation of the chromophore. It is believed that this relativeshifting of UV absorbance as a result of ionization or protonationallows the ratio of UV absorbance at these wavelengths to correlate tothe pH of the pulp effluent.

According to a preferred embodiment, the effluent 6 is filtered first bya coarse filter (1 mm mesh) and then filtered by a 0.2 μm filter (Mott700-1/2-24-0.5) filter to remove colloidal material from the pulpeffluent. If the colloidal material is not removed from the effluent,the resultant turbidity may offset the UV absorbance and affect the UVratios of the present invention.

The pulp bleaching liquor is then diverted into a sample cell, such as aHelma 170-QS 0.2 mm sample cuvette. The cuvette is illuminated by axenon strobe light, such as an Ocean Optics PX-1. The resultant lightoutput that is not absorbed is channelled through a fibre optic cable,such as a 400 μm UV/VIS fibre optic cable, to a UV spectrophotometer,such as an Ocean Optics PC1000 which has been optimized for measuringlight between 200 and 450 nm, the output of which is fed to computer 12.

Computer 12 calculates a mathematical relationship between two or moreUV measurements, which is a ratio or combination of ratios in thispreferred embodiment. Moreover, computer 12 stores in its memory apredetermined interrelationship between the ratio and a characteristicof the effluent.

For example, FIG. 2 shows the predetermined interrelationship of theratio of ultraviolet light absorption at 230 nm and 300 nm (A₂₃₀ nm/A₃₀₀ nm) and residual peroxide in the effluent. The residual peroxidemay be expressed as a residual weight percentage value of the dry pulpweight of the pulp effluent. FIG. 2 shows this predeterminedinterrelationship for two types of wood pulp, spruce and aspen, both ofwhich illustrate a substantially linear interrelationship. As will beappreciated by those skilled in the art, FIG. 2 may be created in thelaboratory by repeated measurements of UV light absorption ratios andthe weight percentage value of H₂ O₂.

Computer 12 acts as a comparing means for comparing the ratio with thepredetermined interrelationship of FIG. 2. This comparison stepdetermines the actual empirical value of the residual weight percentageof H₂ O₂, the characteristic of the effluent being monitored.

According to this preferred embodiment, computer 12 determines whetherto send a feedback signal 14 to control valve 16 to adjust the amount ofH₂ O₂ being fed into the bleaching process taking place in bleachingchamber 4. Thus, computer 12 together with control valve 16 act as ameans for adjusting the amount of input component, such as H₂ O₂,according to the value of the characteristic of the effluent 6, H₂ O₂concentration, which was determined by the ratio of the two UVabsorption measurements.

Thus, as shown by the preferred embodiment of the invention, shown inFIGS. 1 and 2, the disclosed invention permits a real time feedbackcontrol of a pulp bleaching process. The feedback system providessufficient H₂ O₂ to the pulp bleaching process without producingexcessive waste residual H₂ O₂ or pollution.

It is contemplated that other input components, such as NAOH, MgSO₄, ora chelating agent, such as DTPA (diethylenetriamine pentaacetic acid),may added to the bleaching reaction in reduced or increased amounts inaccordance this feedback system.

FIG. 3 shows a graphical representations of the predeterminedinterrelationships of the ratio of UV light absorption at 230 nm and 250nm (A₂₃₀ nm /A₂₅₀ nm) and the pH value of the pulp effluent fordifferent reaction times. As shown, the wavelengths chosen for measuringUV absorption are 230 nm and 250 nm. Thus, the monitoring of the pH ofpulp effluent can be accomplished by comparing the predeterminedinterrelationship shown in FIG. 3 with a real time UV absorption ratiomeasured from the pulp effluent.

FIG. 4 shows a graphical representation of the interrelationship of a UVabsorbance ratio and pulp brightness. In this case the ratio of UVabsorption measurements is A₂₈₀ nm /A₃₅₀ nm. It is contemplated that theincrease in the ratio corresponds to a reduction in the amount ofUV-light absorbing carbonyl containing components in the bleached pulpeffluent. Brightness was standardized using pulp brightness measuringprocedures which are well known in the art and are described in detailin procedure method TAPPI T 452 om-92 entitled Brightness of Pulp,Paper, and Paperboard. According to this standard method, it isunderstood that pulp brightness may be expressed as a percentage of thereflectance of magnesium oxide based upon standard optical propertyprocedures. Further, as stated above, other useful UV absorptionwavelength ratios for measuring pulp brightness have been found toincluding 280 nm/300 nm and 230 nm/250 nm.

As stated earlier, UV absorption wavelength ratios including 280 nm/350nm have been found useful for monitoring the yellowness of the pulp.Standard pulp yellowness measuring procedures are well known in the artand are described in further detail in procedure method TAPPI T524 om-86entitled L, a, b, 45° 0° Colorimetry of White and Near-White Paper andPaperboard.

FIGS. 5, 6, and 7 were obtained show the predictive value of using theUV absorbance ratios of the present invention with for measuringbrightness, residual peroxide, and delignification efficiency. To obtainthese graphs, pulp was bleached under different pH conditions. A sampleof pulp effluent was removed from the pulp at 3 minutes (3 min.),filtered, and then the UV absorbance of the filtered effluent wasmeasured. A series of five different UV ratios were obtained as afunction of pH at three minutes into the bleaching reaction (A₂₃₀ nm/A₂₅₀ nm, A₂₃₀ nm /A₃₀₀ nm, A₃₅₀ nm /A₂₈₀ nm, A₃₀₀ nm /A₂₈₀ nm, A₃₅₀ nm/A₃₀₀ nm). The final pulp brightness, residual peroxide, anddelignification efficiency of the reaction was then measured after 90minutes into the bleaching reaction. Multiple regression analysis, usinga forward stepwise multiple regression, was conducted using these UVratios to determine the best combination of these ratios andcoefficients, (i.e. a predetermined relationship) which best predictedthe final property of the pulp (i.e. optical properties and otherdescriptors of bleaching including delignification efficiency andresidual peroxide). As shown in the graphs, the present invention canuse this predetermined relationship to determine a pulp property (finalbrightness, delignification efficiency, or residual peroxide) using aminimum number of UV light absorbance ratio measurements obtained early(at 3 min.) in the bleaching process.

FIG. 5 shows a graphical representation of actual and predicted valuesof the brightness of spruce wood pulp as a function of pH of spruce woodpulp. A predetermined empirical relationship of brightness as a functionof pH was created using effluent extracted at 3 minutes after initiationof the brightening/bleaching reaction using H₂ O₂. As discussed, thisdata was analyzed by standard forward stepwise multiple regression topredict brightness at 90 minutes after the bleaching reaction started.The results of the forward stepwise multiple regression analysisindicates that the predicted brightness at 90 minutes after initiationof the bleaching/brightening reaction fits the general equation:

    Brightness(@ 90 min.)=a+b*A.sub.230 nm /A.sub.250 nm +c*A.sub.350 nm /A.sub.280 nm +d*A.sub.230 nm /A.sub.300 nm

Multilinear regression provided the following coefficients:

    Brightness(@ 90 min.)=95.75+44.99A.sub.230 nm /A.sub.250 nm -322.6A.sub.350 nm /A.sub.280 nm -4.980A.sub.230 nm /A.sub.300 nm

As shown in this equation, the best relationship is given by combiningthe above three UV ratios. This equation was found to be useful inpredicted the final property, brightness, using UV absorbances obtainedfrom the pulp effluent at 3 minutes after initiation of the bleachingreaction. FIG. 5 shows a high level of reliability between the predictedand actual values of brightness at different pH values.

FIG. 6 shows a graphical representation of actual and predicted valuesof the residual H₂ O₂ (expressed as a percentage weight) in spruce woodpulp as a function of the pH of spruce wood pulp. A predeterminedempirical relationship of residual H₂ O₂ as a function of pH was createdusing effluent extracted at 3 minutes after initiation of thebrightening/bleaching reaction using H₂ O₂. As discussed, this data wasanalyzed by standard forward stepwise multiple regression to predictresidual H₂ O₂ at 90 minutes after the bleaching reaction started. Theresults of the forward stepwise multiple regression analysis indicatesthat the predicted residual H₂ O₂ at 90 minutes after initiation of thebleaching/brightening reaction fits the general equation:

    Residual H.sub.2 O.sub.2 (@ 90 min.)=a+b*A.sub.230 nm /A.sub.250 nm +c*A.sub.350 nm /A.sub.280 nm +d*A.sub.350 nm /A.sub.300 nm

Multilinear regression provided the following coefficients:

    Residual H.sub.2 O.sub.2 (@ 90 min.)=511+104.24A.sub.230 nm /A.sub.250 nm -5130.7A.sub.350 nm /A.sub.280 nm +1521.3A.sub.350 nm /A.sub.300 nm

As shown in this equation, the best relationship is given by combiningthe above three UV ratios. FIG. 6 shows a high level of reliabilitybetween the predicted and actual values of residual H₂ O₂ at differentpH values.

FIG. 7 shows a graphical representation of actual and predicted valuesof the delignification efficiency of spruce wood pulp as a function ofthe pH of spruce wood pulp. A predetermined empirical relationship ofdelignification efficiency as a function of pH was created usingeffluent extracted at 3 minutes after initiation of thebrightening/bleaching reaction using H₂ O₂. As discussed, this data wasanalyzed by standard forward stepwise multiple regression to predictdelignification efficiency at 90 minutes after the bleaching reactionstarted. The results of the forward stepwise multiple regressionanalysis indicates that the predicted delignification efficiency at 90minutes after initiation of the bleaching/brightening reaction fits thegeneral equation:

    delignification efficiency(@ 90 min.)=a+b*A.sub.230 nm /A.sub.250 nm +c*A.sub.350 nm /A.sub.280 nm +d*A.sub.300 nm /A.sub.280 nm

Multilinear regression provided the following coefficients:

    delignification efficiency (@ 90 min.)=151.01+62.253A.sub.230 nm /A.sub.250 nm -772.08A.sub.350 nm /A.sub.280 nm -94.107A.sub.300 nm /A.sub.280 nm

As shown, the best relationship is given by combining the above three UVratios. FIG. 7 shows a high level of reliability between the predictedand actual values of delignification efficiency at different pH values.

Similar experiments using the same bleaching reaction conditions havealso been obtained for aspen pulp. The results of the forward stepwisemultiple regression analysis indicates that the predicted brightness,residual H₂ O₂, and delignification efficiency at 90 minutes afterinitiation of the bleaching/brightening reaction fits the followingequations:

    Brightness(@ 90 min.)=78.33-86.56A.sub.350 nm /A.sub.280 nm +9.1 57A.sub.230 nm /A.sub.280 nm

    Residual H.sub.2 O.sub.2 (@ 90 min.)=-376.24+239.3A.sub.230 nm /A.sub.250 nm.

    Delignification efficiency (@ 90 min.)=31.10-17.73A.sub.350 nm /A.sub.300 nm

It will be appreciated that while the embodiments of FIGS. 1 to 7 teachthe use of some UV absorption wavelength ratios to determine specificproperties an industrial wood pulp bleaching process, it is contemplatedthat other combinations of the disclosed absorbance ratios may also beused to monitor and control various other characteristics in the pulpbleaching process. Also, the disclosed absorbance ratios may undergofurther statistical analysis including multiple regression analysis todetermine other useful mathematical relationships which permit themonitoring and control of various characteristics of the pulp bleachingprocess.

The above-described embodiments of the invention are intended to beexamples of the present invention and alterations and modifications maybe affected thereto, by those of skill in the art, without departingfrom the scope of the invention which is defined solely by the claimsappended hereto.

I claim:
 1. A method for measuring a characteristic of a processeffluent comprising the steps of:(a) irradiating at least a portion ofthe effluent with ultraviolet-visible light for allowing an absorptionof said light by said effluent (b) obtaining at least three measurementsof the ultraviolet-visible light from the effluent, each at a differentwavelength by obtaining:a first measurement at a first wavelength, asecond measurement at a second wavelength, and a third measurement at athird wavelength; (c) formulating at least two ratios from the at leastthree measurements; and (d) using the at least two ratios for computingan empirical value of the characteristic of said effluent.
 2. A methodas defined in claim 1 wherein the empirical value of the characteristicis computed from a predetermined interrelationship including the atleast two ratios.
 3. A method for measuring a characteristic of aneffluent as defined in claim 2 further comprising the step ofcontrolling the characteristic using feedback control for adjusting atleast one feed input component in accordance with the computed empiricalvalue of the characteristic for obtaining a target measurement of thecharacteristic.
 4. A method as defined in claim 3 wherein said effluentis produced by a wood pulp bleaching process.
 5. A method as defined inclaim 4 wherein said wood pulp bleaching process utilizes a peroxidebleaching agent.
 6. A method as defined in claim 5 wherein the first andthe second wavelength is selected from the region between 230 nm to 260nm and the third wavelength is selected from the region between 280 nmto 500 nm.
 7. A method as defined in claim 5 wherein the first, thesecond, and the third wavelength are selected from the group consistingof 230 nm, 280 nm and 300 nm and said characteristic is the amount ofperoxyl anion in the effluent.
 8. A method as defined in claim 4 whereinsaid wood pulp bleaching process utilizes a bleaching agent selectedfrom the group consisting of hydrogen peroxide (H₂ O₂), ClO₂, Cl₂, O₃,and Na₂ S₂ O₄.
 9. A method as defined in claim 3 wherein said effluentis produced by a wood pulp delignification process.
 10. A method asdefined in claim 9, wherein said wood pulp delignification processutilizes a delignification agent selected from the group consisting offNAOH, Na₂ S, O₂, Na₂ SO₃, and enzymes including ligninase, xylanase,mannanase, laccase, and manganese peroxidase.
 11. A method as defined inclaim 3 wherein the first, the second, and the third wavelength areselected from the group consisting of 230 nm, 250 nm, 280 nm, and 300 nmand said characteristic is pH.
 12. A method as defined in claim 3wherein the first, the second, and the third wavelength are selectedfrom the group consisting of 230±5 nm, 250±5 nm, 280±10 nm, 300±10 nm,and 350±5 nm and said characteristic is a degree of ionization ofphenolic groups in the effluent.
 13. A method as defined in claim 3wherein the first, the second, and the third wavelength are selectedfrom the group consisting of 230 nm, 250 nm, 280 nm, 300 nm, and 350 nm,and said characteristic is pulp brightness.
 14. A method as defined inclaim 3 wherein the first, the second, and the third wavelength areselected from the group consisting of 230 mn, 250 nm, 280 nm, 300 nm,and 350 nm and said characteristic is residual hydrogen peroxide in theeffluent.
 15. A method as defined in claim 3 wherein the first, thesecond, and the third wavelength are selected from the group consistingof 230 nm, 250 nm, 280 nm, 300 nm, and 350 nm and said characteristic isa measure of bleaching efficiency.
 16. A method as defined in claim 3wherein the first, the second, and the third wavelength are selectedfrom the group consisting of 270 nm to 280 nm, 330 nm, 350 nm, 400 nm,and 450 nm and said characteristic is pulp yellowness.
 17. A method asdefined in claim 3 wherein said feed input component is selected fromthe group consisting of H₂ O₂, NAOH, MgSO₄, and DTPA (diethylenetriaminepentaacetic acid).
 18. A method as defined in claim 3 wherein thepredetermined interrelationship is described by a following equation:

    empirical value of characteristic=a+b*ratio1+c*ratio2

wherein a, b, and c are factors determined by regression methods andwherein ratio1 and ratio2 each include two measurements at two differentwavelengths selected from the group consisting of the first wavelength,the second wavelength and the third wavelength.
 19. A method as definedin claim 3 wherein said predetermined interrelationship is determined byregression methods.
 20. An apparatus for measuring a characteristic ofan effluent of a process comprising:(a) an ultraviolet-visible detectionunit for allowing a measurement of said effluent, saidultraviolet-visible detection unit for obtaining at least threemeasurements of light in an ultraviolet-visible region, a firstmeasurement at a first wavelength, a second measurement at a secondwavelength and a third measurement at a third wavelength; (b) aprocessor for receiving and processing data from the ultraviolet-visibledetection unit programmed for performing the following steps:formulating at least two ratios from the at least three measurements andusing the at least two ratios for computing an empirical value of thecharacteristic of said effluent.
 21. An apparatus as defined in claim 20wherein the empirical value of the characteristic is computed from apredetermined interrelationship including the at least two ratios. 22.An apparatus for measuring a characteristic of an effluent as defined inclaim 21 further comprising a means for controlling the characteristicusing feedback control for adjusting at least one feed input componentin accordance with the computed empirical value of the characteristicfor obtaining a target measurement of the characteristic.
 23. Anapparatus as defined in claim 22 wherein said effluent is produced by awood pulp bleaching process.
 24. An apparatus as defined in claim 23wherein said wood pulp bleaching process utilizes a peroxide bleachingagent.
 25. An apparatus as defined in claim 24 wherein said first andsecond wavelength are selected from the region between 230 nm to 260 nmand said third wavelength is selected from the region between 280 nm to500 nm.
 26. An apparatus as defined in claim 24 wherein the first, thesecond, and the third wavelength are selected from the group consistingof 230 nm, 280 nm, and 300 nm and said characteristic is an amount ofperoxyl anion in the effluent.
 27. An apparatus as defined in claim 23wherein said wood pulp bleaching process utilizes a bleaching agentselected from the group consisting of hydrogen peroxide (H₂ O₂) ClO₂,Cl₂, O₃, and Na₂ S₂ O₄.
 28. An apparatus as defined in claim 22 whereinsaid effluent is produced by a wood pulp delignification process.
 29. Anapparatus as defined in claim 28 wherein said wood pulp delignificationprocess utilizes a delignification agent selected from the groupconsisting of NAOH, Na₂ S, O₂, Na₂ SO₃, and enzymes including ligninase,xylanase, mannanase, laccase, and manganese peroxidase.
 30. An apparatusas defined in claim 22 wherein said first, second, and third wavelengthare selected from the group consisting of 230 nm, 250 nm, 280 nm and 300nm and said characteristic is pH.
 31. An apparatus as defined in claim22 wherein said first, second, and third wavelength are selected fromthe group consisting of 230±5 nm, 250±5 nm, 280±10 nm, 300±10 nm, and350±5 nm, and said characteristic is a degree of ionization of phenolicgroups in the effluent.
 32. An apparatus as defined in claim 22 whereinthe first, the second, and the third wavelength are selected from thegroup consisting of 230 nm, 250 nm, 280 nm, 300 nm, and 350 nm and saidcharacteristic is pulp brightness.
 33. An apparatus as defined in claim22, wherein the first, the second, and the third wavelength are selectedfrom the group consisting of 230 nm, 250 nm, 280 nm, 300 nm, and 350 nmand said characteristic is residual hydrogen peroxide in the effluent.34. An apparatus as defined in claim 22 wherein the first, the second,and the third wavelength are selected from the group consisting of 230nm, 250 nm, 280 nm, 300 nm, and 350 nm and said characteristic is ameasure of bleaching efficiency.
 35. An apparatus as defined in claim 22wherein the first, the second, and the third wavelength are selectedfrom the group consisting of 270 nm to 280 nm, 330 nm, 350 nm, 400 nm,and 450 nm and said characteristic is pulp yellowness.
 36. An apparatusas defined in claim 22 wherein said feed input component is selectedfrom the group consisting of H₂ O₂, NAOH, MgSO₄, or DTPA(diethylenetriamine pentaacetic acid).
 37. An apparatus as defined inclaim 22 wherein the predetermined interrelationship is described by afollowing equation:

    empirical value of the characteristic=a+b*ratio1+c*ratio2

wherein a, b, and c are factors determined by regression methods andwherein ratio1 and ratio2 each include two measurements at two differentwavelengths selected from the group consisting of the first wavelength,the second wavelength and the third wavelength.
 38. An apparatus asdefined in claim 21 wherein said predetermined interrelationship isdetermined by regression methods.
 39. A method for measuring acharacteristic of an effluent from a mechanical pulp bleaching processcomprising the steps of:a) irradiating at least a portion of theeffluent with ultraviolet-visible light for allowing an absorptionmeasurement of said effluent; b) obtaining at least three measurementsof UV-visible light from the effluent, a first measurement at a firstwavelength, a second measurement at a second wavelength, and a thirdmeasurement at a third wavelength; c) formulating at least two ratiosfrom the at least three measurements; and d) using the at least tworatios for computing an empirical value of the characteristic of saideffluent.
 40. A method as defined in claim 39 wherein the empiricalvalue of the characteristic is described by a following equation:

    empirical value of characteristic=a+b*ratio1+c*ratio2

wherein a, b, and c are factors determined by multiple regressionanalysis and wherein ratio1 and ratio2 each include two measurements attwo different wavelengths selected from the first wavelength, the secondwavelength or the third wavelength.
 41. A method as defined in claim 40wherein the characteristic of the effluent is related to a samecharacteristic of the mechanical pulp.
 42. A method as defined in claim40 wherein the characteristic is one of a bleaching efficiency, adelignification efficiency, pulp yellowness, pulp brightness, pH, adegree of ionization of phenolic groups in the mechanical pulp effluent,an amount of peroxyl anion in the mechanical pulp effluent, and anamount of residual hydrogen peroxide in the mechanical pulp effluent.